1. Field of the Invention
The present invention relates to an image reading apparatus and method and, more particularly, to an apparatus and method for reading the image of a document in a time-series in a facsimile device, image scanner, digital copier, electronic blackboard, and the like.
2. Description of the Related Art
In recent years, image reading devices commonly known as contact-type image sensors have generally been utilized in document readers such as facsimile devices, instead of the charge coupled device (CCD)-type image reader for which reducing optics are necessary. Since this image reading device is formed as a one-dimensional plurality of photoelectric energy converting elements made of a thin film semiconductor, such as amorphous silicon on a glass substrate, the image of a document can be read uniformly. A normal photodiode is used in the photoelectric energy converting element, but because of the extremely weak photocurrent generated in the photodiode, a charge storage method is widely employed in which this photocurrent is detected from the charge temporarily accumulated in the junction capacitance of the photodiode. Moreover, as a result of the number of photoelectric energy converting elements becoming extremely large, a matrix driver, which may have a smaller number of switching elements, is usually employed.
For example, as shown in FIG. 1, a prior art image reading device is constructed by fabricating a one-dimensional array of m by n photoelectric energy converting elements each comprising a photodiode 1 and a blocking diode 2 connected to this photodiode 1 in series and with opposite polarity. These formed pairs of photodiodes 1 and blocking diodes 2 are divided every m diode pairs into n blocks, B.sub.1, B.sub.2, . . . , B.sub.n. The anode terminals of the blocking diodes 2 in each block B.sub.1, B.sub.2, . . . , B.sub.n are connected commonly via a respective buffer gate 3 to the respective output terminal of a shift register 4. The anode terminals of the photodiodes 1 in the same relative positions in the respective blocks B.sub.1, B.sub.2, . . . , B.sub.n are connected together commonly via a respective current amplification circuit IV.sub.1, IV.sub.2, . . . , IV.sub.m to a respective integration circuit IN.sub.1, IN.sub.2, . . . , IN.sub.m. Moreover, the integration circuits IN.sub.1, IN.sub.2, . . . , IN.sub.m are connected to sample holding circuits SH.sub.1, SH.sub.2, . . . , SH.sub.m, a multiplexer circuit MPX and an amplification circuit 5, so that a signal processing circuit for time-integrating the electric currents I.sub.1, I.sub.2, . . . , I.sub.m flowing out from the photodiodes 1 is constructed by the current amplification circuits IV.sub.1, IV.sub.2, . . . , IV.sub.m, integration circuits IN.sub.1, IN.sub.2, . . . , IN.sub.m, sample holding circuits SH.sub.1, SH.sub.2, . . . , SH.sub.m, multiplexer circuit MPX and amplification circuit 5.
According to this image reading device, as shown in the timing diagram of FIG. 2, a data input pulse Din which is input into the shift register 4 is shifted in sequence within the shift register 4 in accordance with a clock pulse CLK, and is output in sequence from the respective output terminal. Thus, a driving voltage is applied to the photodiodes 1 in the block units B.sub.1, B.sub.2, . . . , B.sub.n from one block to the next. In the photodiodes 1 to which the driving voltage is applied, a current I.sub.1, I.sub.2, . . . , I.sub.m flows in proportion to the photo signal stored in the junction capacitance, and this is amplified by the respective current amplified circuit IV.sub.1, IV.sub.2, . . . , IV.sub.m. Further, by using the signal processing circuit formed by the integration circuit IN.sub.1, IN.sub.2, . . . , IN.sub.m, sample holding circuit SH.sub.1, SH.sub.2, . . . , SH.sub.m, multiplexer circuit MPX and amplification circuit 5, the current I.sub.1, I.sub.2, . . . , I.sub.m flowing out from the photodiode 1 is signal processed, to give an output voltage Vout. Thus, the electrical signals of the respective photodiodes 1 are scanned in the block units B.sub.1, B.sub.2, . . . , B.sub.n from one block to the next by using the shift register 4 and so forth, and the channel parts within one block are read out simultaneously.
As explained above, the shift register output, as shown in the timing diagram of FIG. 2, serves as the driving voltage Vd of the matrix driver using the charge storage method. As a result, for example, since there are 1728 elements in an 8 element/mm image reading device of A4 size, either 32 channels by 54 blocks, 16 channels by 108 blocks or 8 channels by 216 blocks are constructed, of which the construction is usually 16 channels by 108 blocks. However, there is a problem in that a large number of shift registers is necessary in each of these constructions. Furthermore, since in the case of forming an IC, analog circuit components are expensive, it is necessary to use many digital circuits in the construction.